Pyrolysis is a well-known process in which carbon-containing substances, often called a “biomass”, such as agricultural by products, wood chips, human sewage, etc., are heated in the absence of oxygen to several hundred degrees Celsius. Without oxygen, the material does not burn. Instead, the carbon-based compounds separate into three distinct products—a solid, called “char”, a combustible liquid, called “bio-oil”, and a mixture of gasses such as hydrogen H2, carbon monoxide CO and carbon dioxide CO2, also known as “syngas”. Most of the products of the pyrolysis reaction are combustible, and therefore pyrolysis is a process that converts what had simply been waste into useable fuels.
Pyrolysis occurs in the absence of oxygen, and therefore must be carried out in a special reactor chamber. The process can occur in a vacuum, or in the presence of gases such as water/steam, nitrogen or argon. The biomass can also be mixed with particles, such as sand, and stirred to increase the exposed surface area.
The proportion of the reaction products depends on several factors including the composition of the biomass and the process parameters. In some processes, the yield of bio-oil is optimized when the pyrolysis temperature is around 500° C. and the heating rate is high (i.e. 1,000° C./s). This is often called “fast pyrolysis”. Processes that use slower heating rates are called “slow pyrolysis”, and bio-char is usually the major product of such processes. Table I compares the properties of several types of pyrolysis and their reaction products. This table is adapted from Table 8-12 in the reference book by Donald L. Klass [Biomass for Renewable Energy, Fuels, and Chemicals, Academic Press, San Diego (1998)]. Pyrolysis is an active area research and development, and more can be found in texts such as the Applied Pyrolysis Handbook, edited by Thomas P. Wampler [CRC Press, Boca Raton, Fla. (2006)], or journals such as the Journal of Analytical and Applied Pyrolysis, edited by D. Fabbri, K. J. Voorhees and published by Elsevier (Amsterdam, NL).
TABLE ITypical Biomass Pyrolysis Technologies, Conditions and MajorProducts (adapted from Biomass for Renewable Energy, Fuels,and Chemicals by D. L. Klass).ResidenceHeatingTemperatureMajorTechnologyTimeRate(° C.)ProductsConventionalHours-daysVery low300-500CharcoalCarbonizationPressurized15 min-2 h Medium450CharcoalCarbonizationConventionalHoursLow400-600Char-oil &PyrolysisSyngas5-30minMedium700-900Biochar &SyngasVacuum2-30secMedium350-450OilPyrolysisFlash0.1-2secHigh400-650OilPyrolysis<1secHigh650-900Oil &Syngas<1secVery High1000-3000Syngas
Pyrolysis is an endothermic process, and so a source of heat must be supplied. Typically, for a pyrolysis facility on the surface of the Earth, the heat is supplied by burning natural gas or some other fuel to heat a reactor chamber. In some cases, the syngas produced by the reaction is cycled back to provide additional fuel for the pyrolysis reactor.
Another source of heat lies beneath the surface of the Earth, in the form of geothermal energy. With the core of the Earth believed to be over 5,000° C., there is enough heat stored from the original formation of the Earth and generated by ongoing radioactive decay to provide all the energy mankind can use.
The usual problems encountered in attempting to utilize geothermal energy have been practical ones of access, since the surface of the Earth is much cooler than the interior. The average geothermal gradient is about 25° C. for every kilometer of depth. This means that the temperature at the bottom of a well 5 km deep can be expected to be at a temperature of 125° C. or more. Oil companies now routinely drill for oil at these depths, and the technology required to create holes of this magnitude in the Earth is well known. (The deepest oil well at this time is over 12 km deep.) Wells of this depth, however, can be very expensive, costing over $10M to drill.
However, near geological fault zones, fractures in the Earth's crust allow magma to come much closer to the surface. This gives rise to familiar geothermal landforms such as volcanoes, natural hot springs, and geysers. In the seismically active Long Valley Caldera of California, magma at a temperature more than 700° C. is believed to lie at a depth of only 6 km. Alternatively, if lower temperatures can be utilized, a well dug to a depth less than 1 km in a geothermal zone can achieve temperatures over 100° C. A well 1 km deep often can cost much less than $1M to drill.
It may, however, be unnecessary to drill a well of any kind. The worldwide search for oil has left a multitude of holes in the Earth, many going deep enough to tap into a significant source of heat. For these wells, all only surface infrastructures need be supplied to allow this source of heat to be tapped.
In a previous patent application entitled GEOTHERMAL ENERGY COLLECTION SYSTEM, U.S. patent application Ser. No. 13/815,266, submitted on Feb. 14, 2013 and incorporated herein in its entirety by reference, inventions by David Alan McBay, the inventor of the inventions disclosed here, are presented. These disclosed inventions comprise a system in which a thermal mass is lowered into a well to a Heat Absorption Zone, which will typically be a stratum of the Earth geothermally heated to 350° C. or more. While in this Zone, the temperature of the thermal mass rises because it is surrounded by the Earth's heat. Once hot, this thermal mass is then raised again to the surface, and the heat transferred in a Heat Transfer Zone to a suitable means for driving an industrial process, such as the generation of electricity or powering a chemical reaction.
A facility designed to lower and raise thermal masses according to the previous invention can also serve as a facility to carry out pyrolysis, assuming that the temperature in the Heat Absorption Zone is hot enough to drive the desired pyrolysis reaction, and assuming that a suitable reactor for pyrolysis can be suspended from a cable and lowered into the Heat Absorption Zone.
There is therefore a need to have an apparatus comprising a reactor for pyrolysis that can be raised and lowered on a cable into a well shaft and used with a source of geothermal heat.
An apparatus to drill wells deep enough to perform high temperature pyrolysis may present additional complications. Drilling into the Earth, especially for oil exploration, has developed significantly over the previous century. From the initial rotary rock bit developed in Texas in the 1900s to the more advanced tricone bits in the mid century, improvements have been made both in design and in materials for fabrication.
These drilling technologies, however, still rely on the friction of metal against rock, and use force from above as well as cutting and pinching motions in the bit itself to break away pieces of the rock being penetrated. This can be fine for softer soils and rock, but for drilling through harder layers, such as granite, the drill bits quickly wear out and break, and must be withdrawn and replaced for drilling to continue.
Drilling techniques that induce spallation have therefore recently been proposed. These involve the rapid and sudden heating of the surface of the rock in the borehole. The sudden temperature gradient creates stress fractures in the rock, and continued application of heat causes rock fragments, called spalls, to break off. Continued application of the heat allows the hole to be drilled without significant grinding or mechanical effort.
The initial spallation drilling techniques used open flames to create the temperature gradient, but a flame cannot be sustained in a borehole filled with mud or water. The recent development of Potter Drilling, as described in U.S. Pat. No. 8,235,140, (METHODS AND APPARATUS FOR THERMAL DRILLING, filed by inventors T. Wideman, J. Potter, D. Dreesen, and R. Potter and assigned to Potter Drilling, inc. of Redwood City, Calif.) involves directing a hot fluid, such as water, with a temperature about 500° C. above the ambient temperature of the material, onto a surface of the material being drilled. Spallation occurs, regardless of whether oxygen is present in the hole. After breaking away, the spalls are then pumped to the surface along with the used water from the process.
Although Potter Drilling has been demonstrated, there are some problems with the system. Most notably, providing a source of 500° C. fluid from the surface and insuring that its temperature does not drop as it travels down a well that can be kilometers deep requires a special tubing system capable of high temperatures and pressures. Likewise, energy must be expended pumping the spalls and spent water form the system.
There is therefore a need for a spallation system that has a local heating mechanism, and a local storage system for the spalls and debris that are created while drilling wells deep enough in rock that is hot enough to be suitable for efficient pyrolysis.
If a source of magma, such as a lava dome, can be tapped through a geothermal well, variations on the usual pyrolysis reactions have been observed. In particular, the carbon-based biomass reacts chemically with the minerals in the magma. Because of the high temperatures involved, the reaction products favor the production of gasses, including hydrogen H2, carbon monoxide CO, carbon dioxide CO2, and methane CH4. When the biomass is mixed with water, or has a naturally high water content, large amounts of steam are also generated.
However, access to a lava dome is not an everyday occurrence. Very high temperatures are involved, and if magma is to be used as a heat source, the biomass to be converted must be supplied in a controlled manner.
There is therefore a need to have an apparatus comprising a means for facilitating pyrolysis that can interact with a lava dome.